1. Field of the Invention
This invention relates to temperature control in systems wherein a device under test (DUT) is thermally conditioned (heated or cooled to some thermal state) by a thermal conditioning device that adds or removes heat to/from the device by convection, conduction, or radiation. More particularly, this invention discloses a method and apparatus for dynamically determining an optimum temperature profile for the conditioning device such that the DUT is conditioned as quickly as possible without exposing either the conditioning device or the DUT to unacceptable temperatures.
2. Description of Related Technology
The nature of heat transfer is such that a differential in temperature between two masses must exist before heat will flow between them. The greater the temperature differential, the greater the heat flow will be. This phenomenon operates equally for masses that are separate but adjacent and for masses that are adjacent parts of a monolithic whole.
The rate of transfer of heat within a mass having an internal temperature differential is regulated by that substance""s resistance to heat flow; its thermal conductivity. Every substance exhibits a different and predictable thermal conductivity.
It follows that to change the temperature of the center of a mass (the xe2x80x9ccorexe2x80x9d) to some desired temperature, the outside surface of the mass (the xe2x80x9cskinxe2x80x9d) must be exposed to a temperature beyond the desired core temperature for a time period adequate to allow the sufficient transfer of heat given the mass"" thermal conductivity.
The foregoing concept is clearly illustrated by the everyday example of roasting meat within a conventional oven. The meat is roasted for a given period of time, as determined by its weight, at a comparatively high oven air temperature in order to achieve a desired lower internal or xe2x80x9ccorexe2x80x9d temperature. The differential temperature causes heat to flow to the core of the meat, thereby raising its temperature.
As previously stated, the transfer of heat into or out of the core of a mass consumes a finite amount of time. This time has value, so there is an incentive to achieve the thermal objective (e.g., the desired core temperature) as quickly as possible. A simple solution to accelerating the heat transfer is to increase the temperature differential between the object""s skin and it""s core. The greater differential will result in faster heat transfer.
However, it will be appreciated that many objects to be heated or cooled have practical thermal limits that must be respected if the object is to be not damaged. or destroyed by the heating or cooling process. The most common limits that must be considered are the maximum and minimum temperatures that the skin of the mass can tolerate, and the maximum skin to core temperature differential (thermal stress) that can be tolerated.
Therefore, there is a limit to the amount of heat that can be added or removed from the skin of a DUT during the heating or cooling process without exceeding the thermal limits of the object. Controlling the temperature of the skin of a DUT to that limit will allow the maximum rate of heat transfer to/from the object""s core while still respecting the limits of the object""s skin. If there is a thermal differential limit as well, then the skin temperature may have to be further restrained to remain within that limit.
Another factor that must be considered is the so-called xe2x80x9clatencyxe2x80x9d of the heating or cooling process. As discussed in greater detail below, if the skin of a DUT is subjected to a more extreme temperature than that desired in the object""s core until such time as the core achieves the desired temperature, then the core will be at the desired temperature but the skin will be at a more extreme temperature with the mass between the two areas having a temperature gradient therebetween. If no more heat is added or removed, the entire mass will then equalize in temperature over time. The equalized temperature will be more extreme than the core temperature desired.
Referring again to the example of roasting meat, if a given internal or core temperature is desired, and the meat is roasted at a higher temperature than the desired core temperature, the oven may be turned off when the core temperature has reached a value somewhat less than the desired value. After the oven is turned off, the core temperature will climb to the desired value while the skin region transfers the last of its excess heat to the core in the process of thermal equalization. It should be noted, however, that while this approach may be useful in roasting meat where the allowable tolerances are comparatively high, it is not useful in most thermal conditioning applications having more limited allowable tolerances, and where there is generally little experiential basis for the applying the technique.
To change the core temperature of a DUT undergoing conditioning, the skin of the object is typically exposed to a conductive or convective controlled temperature mass that transfers heat to/from the skin. It is the temperature of this external mass that must be controlled to achieve the desired heat transfer to/from the core of the object. Due to the thermal conductivity and mass of the object there is often substantial thermal latency in the transfer process. One reliable way to achieve the desired core temperature without xe2x80x9covershootingxe2x80x9d, is to regulate the skin""s thermal environment such that as the object""s core approaches the desired temperature the object""s skin temperature is forced to approach the same temperature. As the desired temperature is reached, the temperature difference between the core region and skin region approaches zero and heat transfer effectively ceases. See FIG. 1, which illustrates the response of an exemplary prior art thermal conditioning system.
The typical prior art method, used for achieving this type of convergent control is to measure the temperature of the thermal environment that acts upon the object""s skin and also measure the temperature of the DUTs core. When determining whether to add or remove heat from the thermal environment, it is the average of the two temperatures that is compared to the temperature objective to make the determination. Thus, the environment will be thermally over-driven by the amount the DUTs core varies from the desired temperature. As the DUTs core approaches the desired temperature, the average of the two temperatures will require that the DUTs environment approach the desired core temperature at the same rate.
The temperature averaging method described above has the substantial disadvantage that it has no method for respecting the thermal limitations of the device in which the thermal environment is created, nor does it respect the thermal limits of the DUT being conditioned. It is quite possible for the averaging method to call for additional heating/cooling when either the skin of the device being conditioned, or the conditioning device itself is already at or beyond its limits. Substantial damage to property and risk to operators results from the unrestrained use of such averaging methods.
Therefore, to make effective use of this type of averaging method, it is imperative that the output from the control system that is using the average temperature to call for heating or cooling be restrained if that output calls for the addition or removal of heat in a manner that would cause the limits of the thermal conditioning device, or the DUT, to be exceeded. If the temperature control system is a simple xe2x80x9con/offxe2x80x9d thermostat type control, externally restraining the control system output will be satisfactory. However, if the control method being used is a more sophisticated method designed around a closed feedback loop that allows the control system to adapt or modify its control output based upon the results of its prior operation, then the external restraining of the control outputs can be disastrous.
Almost all precision temperature control systems involve a method that uses process result feedback in some type of closed loop to adaptively regulate temperature while adjusting for the thermal response of the environment/device being controlled. The feedback loop, and the analysis of the feedback data over time, is the essence of closed loop temperature process control. It is therefore clear that any system that uses such a control method would suffer substantially if its output was externally restrained or xe2x80x9cclipped,xe2x80x9d since the external clipping of the output would result in substantial variation to the result of the control system""s output. To tolerate this kind of modification of the output signal by an external system, the primary control system would have to be fed accurate data as to the magnitude and timing of the clipping. Even if this was done, the system required to cope with this additional data would be substantial and burdensome at best. As a result, xe2x80x9cclippingxe2x80x9d types of control systems are inappropriate for closed loop thermal systems except in very limited situations.
Based on the foregoing, an improved method and apparatus for controlling the temperature of the thermal conditioning device is needed. Such an improved method and apparatus would provide accelerated heat transfer by driving the temperature of the thermal environment beyond the desired conditioned device core temperature, and a mechanism for achieving thermal convergence of the environment and the conditioned device core temperatures to prevent xe2x80x9covershoot.xe2x80x9d These objectives would ideally be accomplished without exceeding the various thermal limits imposed by the conditioning device or the conditioned device; and would not interfere with the control feedback loop of the temperature control system.
The present invention satisfies the aforementioned needs by providing an improved temperature control method and apparatus useful in the thermal conditioning of devices.
In a first aspect of the invention, an improved method of controlling the environmental parameters of a device under test (DUT) is disclosed which incorporates the calculation of a moveable temperature setpoint which will 1) maximize the speed of the thermal test or conditioning routine; 2) respect the limits of the DUT with respect to both absolute skin temperature limits and thermal stress: 3) respect the thermal limitations of the test or conditioning equipment being used; and 4) maximize the thermal uniformity of the DUT when the user""s specified temperature setpoint is reached in the DUT core. In one embodiment, a system operating range (SOR) and DUT operating range (DOR) are calculated based on the thermal and stress limits of the DUT, temperature control system (TCS), and thermal conditioning apparatus. A control setpoint (CSP) which is different than the desired DUT core temperature specified by the user (i.e., the PSP) is then calculated based on the difference between the PSP and the secondary temperature sensing probe input temperature, the value of two predetermined setup parameters, and the relationship between the SOR and DOR, so as to effectuate varying amounts of heat transfer between the thermal conditioning environment and the DUT. As the desired DUT core temperature is approached, movement of the control setpoint is terminated and the differential between core and skin temperature of the DUT reduced accordingly until the user-specified setpoint is reached.
In a second aspect of the invention, a device thermally conditioned using the aforementioned method is disclosed.
In a third aspect of the invention, an algorithm incorporating the method described above is disclosed. In one exemplary embodiment, the computer program is compiled into an object code format which is stored on a magnetic storage medium, and which is capable of being run on a digital computer processor. The algorithm receives inputs (via the host computer system, described below) from instrumentation associated with the thermal conditioning system, such as chamber/device temperature probes, and calculates the Control Setpoint (CSP) which is fed back to the thermal conditioning system to effectuate control of the chamber and device temperature.
In a fourth aspect of the invention, an improved method and algorithm for controlling the temperature differential limits of a device under test (DUT) are disclosed. Specifically, variable differential thermal limits are employed as a function of the core temperature of the DUT in order to control thermal shock to the DUT during various temperature transitions.
In a fifth aspect of the invention, a computer system incorporating the computer program previously described is disclosed. In one embodiment, the computer system comprises a standard microcomputer (personal computer) having a display, magnetic disk drive, microprocessor, internal memory, and input/output port for receiving and transmitting data to and from the computer. The aforementioned computer program is loaded into the internal memory from the storage area and run by the microprocessor to effect temperature control of the DUT. In a second embodiment, a digital processor is integrated with the temperature control system, the above-described computer program being stored within the memory or storage device associated with the processor/TCS.
In a sixth aspect, a thermal conditioning system is disclosed which incorporates the method, computer program, and computer system previously described. In one embodiment, a TCS is operatively coupled to a thermal conditioning chamber having a plurality of temperature probes for measuring the temperature of the conditioning environment as well as that of the DUT. The TCS may be of any compatible configuration including the PID or fuzzy logic types. The computer system previously described is operatively coupled to the TCS, whereby the former receives temperature data and other relevant inputs from the latter, and periodically calculates and provides a control setpoint (CSP) value thereto for control of the thermal conditioning chamber.